Benthic Habitats of Puerto Rico and the U.S. Virgin Islands;
Photomosaic of Puerto Rico (Ceiba), 1999

Abstract:

Habitat maps of Puerto Rico and the U.S. Virgin Islands were created
by visual interpretation of aerial photographs using the Habitat
Digitizer Extension. Aerial photographs are valuable tools for
natural resource managers and researchers since they provide an
excellent record of the location and extent of habitats. However,
spatial distortions in aerial photographs due to such factors as
camera angle, lens characteristics, and relief displacement must be
accounted for during analysis to prevent incorrect measurements of
area, distance, and other spatial parameters.

These distortions of scale within an image can be removed through
orthorectification. During orthorectification, digital scans of
aerial photos are subjected to algorithms that eliminate each source
of spatial distortion. The result is a georeferenced digital mosaic
of several photographs with uniform scale throughout the mosaic.
Features near land are generally georeferenced with greater accuracy
while the accuracy of features away from land is generally not as
good. Where no land is in the original photographic frame only
kinematic GPS locations and image tie points were used to
georeference the images. After the orthorectified mosaics were
created, photointerpreters were able to accurately and reliably
delineate boundaries of features in the imagery as they appear on the
computer monitor.

Planar coordinates are encoded using Coordinate Pair
Abscissae (x-coordinates) are specified to the nearest 2.4
Ordinates (y-coordinates) are specified to the nearest 2.4
Planar coordinates are specified in meters

The horizontal datum used is North American Datum of 1983.
The ellipsoid used is Geodetic Reference System 80.
The semi-major axis of the ellipsoid used is 6378137.
The flattening of the ellipsoid used is 1/298.257.

The National Ocean Service is conducting research to
digitally map biotic resources and coordinate a long-term monitoring
program that can detect and predict change in U.S. coral reefs, and
their associated habitats and biological communities.

Aerial photographs were acquired for Puerto Rico and the U.S. Virgin
Islands Benthic Mapping Project in 1999 by NOAA Aircraft Operation
Centers aircraft and National Geodetic Survey cameras and personnel.
Approximately 600, color, 9 by 9 inch photos were taken of the
coastal waters of Puerto Rico and the U.S. Virgin Islands at 1:48000
scale. Specific sun angle and maximum percent cloud cover
restrictions were adhered to when possible during the photography
mission to ensure collection of high quality imagery for the purpose
of benthic mapping.

Print and diapositives were created from the original negatives.
Diapositives were then scanned at a resolution of 500 dots per inch
(DPI) using a metric scanner, yielding 2.4 by 2.4 meter pixels for
the 1:48000 scale photography. All scans were saved in TIFF format
for the purposes of orthorectification and photointerpretation.

Georeferencing/mosaicing of the TIFF's was performed using Socet Set
Version 4.2.1. Lens correction parameters were applied to each frame
to eliminate image distortion. Airborne kinematic GPS was then used
when available to provide a first order geolocation. When this
information was not available, measurements were made between
flightline strips for input into Socet Set to provide preliminary
co-registration.

Image to image tie-points were then used to further co-register the
imagery, especially for photos taken over open water where ground
control points were not available. Fixed ground features visible in
the scanned photos were selected for ground control points (GCP's)
which were then used to georeference the imagery. GCP's were measured
using real-time DGPS (differential Global Positioning System). Points
were obtained with a wide distribution throughout the imagery,
especially on peninsulas and outer islands whenever possible since
this results in the most accurate registration throughout each image.
Only ground control points for terrestrial features were collected
due to difficulty of obtaining precise positions for submerged
features.

A custom digital terrain model (DTM) was then created using the Socet
Set software to correct for feature displacement due to terrain
effects. To accomplish this, water features and the shoreline were
set to an elevation of zero. Preliminary experimentation revealed
that the effects of refraction on the position of submerged features
in the imagery were not significant enough to make a correction for
underwater displacement according to Snell's law. Selected land
elevation points were then inserted from USGS 1:24000 Digital
Elevation Models or other elevation data sets where clouds or other
sources of interference prevented the Socet Set software from
automatically making an accurate DTM.

Once the terrain models were complete and a draft orthorectified
mosaic was produced, a set of independent ground control points was
used to measure the quality of each mosaic's rectification and ensure
that it met acceptable limits of horizontal spatial accuracy. If
spatial accuracy was not acceptable based on this comparison,
additional modifications were made, until a satisfactory mosaic was
created for each island. In general, mosaics were georeferenced such
that pixels are positioned within one pixel width of their correct
location.

Person who carried out this activity:

National Oceanic and Atmospheric Administration (NOAA), National Ocean Service (NOS), National Ocean Service (NOS), National Centers for Coastal Ocean Science (NCCOS), Center for Coastal Monitoring and Assessment (CCMA), Biogeography Program
Biogeography Team Leader, Mapping Manager
1305 East West Highway, N/SCI-1
Silver Spring, MD 20910

Horizontal accuracy was determined by solution of Socet Set generated
model (RMS less than 1) and by comparison to independent ground control data.
x=0.9 +/-9.5, y=2.6 +/-7.8 values are in meters +/- standard
deviation

No color balancing was attempted since this
alters color and textural signatures in the original imagery and
interferes with the photointerpreter's ability to delineate habitats.
As a result mosaics have visible seams between adjacent photos. This
provides the photointerpreter with "true color" imagery for maximum
ability to identify and delineate benthic features.

Once all of the photographs were
orthorectified, the best segments of each photograph were selected
for creation of the final mosaic. Segments of each photograph were
selected to minimize sun glint, cloud interference, and turbidity in
the final mosaic. Where possible, parts of images obscured by sun
glint or clouds were replaced with cloud/glint free parts of
overlapping images. As a result, most mosaics have few or no clouds
or sun glint obscuring bottom features.

Data are not to be used for navigation.
Disclaimer- While every effort has been made to ensure that these data
are accurate and reliable within the limits of the current state of
the art, NOAA cannot assume liability for any damages caused by any
errors or omissions in the data, nor as a result of the failure of
the data to function on a particular system. NOAA makes no warranty,
expressed or implied, nor does the fact of distribution constitute
such a warranty.